Conventional thermal, flow measuring devices use usually two, as equal as possible, temperature sensors, which are arranged in, most often, pin-shaped, metal shells, so-called stingers, or prongs, and which are in thermal contact with the medium flowing through a measuring tube or through the pipeline. For industrial application, the two temperature sensors usually are installed in a measuring tube; the temperature sensors can, however, also be mounted directly in the pipeline. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. The heating unit is either an additional resistance heater, or, if the temperature sensor itself is a resistance element, e.g. an RTD (Resistance Temperature Device) sensor, such is heated by conversion of electrical power, e.g. by a corresponding variation of the measuring electrical-current. The second temperature sensor is a so-called passive temperature sensor: It measures the temperature of the medium.
Usually, in a thermal, flow measuring device, the heatable temperature sensor is so heated that a fixed temperature difference is established between the two temperature sensors. Alternatively, it has also been known to provide, via a control unit, a constant heating power.
If there is no flow in the measuring tube, then a constant amount of heat is required as a function of time for maintaining the predetermined temperature difference. If, in contrast, the medium to be measured is moving, the cooling of the heated temperature sensor is essentially dependent on the mass flow of the medium flowing past. Since the medium is colder than the heated temperature sensor, the flowing medium transports heat away from the heated temperature sensor. In order thus in the case of a flowing medium to maintain the fixed temperature difference between the two temperature sensors, an increased heating power is required for the heated temperature sensor. The increased heating power is a measure for the mass flow of the medium through the pipeline.
If, in contrast, a constant heating power is supplied, then the temperature difference between the two temperature sensors lessens as a result of the flow of the medium. The particular temperature difference is then a measure for the mass flow of the medium through the pipeline, or through the measuring tube.
There is, thus, a functional relationship between the heating energy needed for heating the temperature sensor and the mass flow through a pipeline, or through a measuring tube. The dependence of the so-called heat transfer coefficient of the mass flow of the medium through the measuring tube, or through the pipeline is utilized in thermal, flow measuring devices for determining the mass flow. Devices, which operate on this principle, are available from Endress+Hauser under the marks, t-switch’, t-trend’ or ‘t-mass’.
Until now, mainly RTD-elements with helically wound platinum wires have been applied in thermal, flow measuring devices. In the case of thin film, resistance thermometers (TFRTDs), conventionally, a meander-shaped platinum layer is vapor deposited on a substrate. Beyond that, another, glass layer is applied for protecting the platinum layer. The cross section of the thin film, resistance thermometer is, in contrast with the round cross section of RTD-elements, rectangular. The heat transfer into the resistance element and/or from the resistance element occurs accordingly via two, oppositely lying surfaces, which together make up a large part of the total surface of a thin film, resistance thermometer.
The patent documents EP 0 024 327 and U.S. Pat. No. 4,083,244 show different embodiments of thermal, flow measuring devices, which can also ascertain flow direction. In this regard, flow conditioning bodies are arranged in the flow before a temperature sensor.